Arrhythmias remain a major health problem, causing at least 250,000 deaths annually in the United States. Pharmacological treatments often do more harm than good, and device therapies are limited by high cost and effects on quality of life. Ion channel mutations cause rare inherited arrhythmopathies, but account for only a small fraction of patients with life-threatening arrhythmias and sudden death. Most arrhythmias occur during myocardial ischemia, following myocardial infarction, and in patients with poor left ventricular (LV) function of any etiology. Aside from ejection fraction (EF), few clinically useful indicators to stratify the risk of sudden death have been identified. The role of subtle differences in ion channel expression and/or structure in predisposing patients to arrhythmias and modulating the risk of sudden death is unknown. In an ischemic cardiomyopathy population, we have found that a common polymorphism in the K+ channel HERG (K897T) worsens survival and increases sudden death. A polymorphism of the beta1-adrenergic receptor (S49G) also modulates the risk of pump failure vs. arrhythmic death. In this proposal, we will prospectively test whether polymorphisms in ion channel and ion channel modifying genes are associated with arrhythmias in a population with internal cardioverter-defibrillators (ICDs) and poor LV function. We will: 1) Directly test the hypothesis that the HERG K897T polymorphism predicts arrhythmia susceptibility in 1700 individuals with an EF below 30 percent and ICD implants. The subjects will be followed prospectively for a period of up to five years with freedom from appropriate ICD shock as the primary endpoint. 2) Test the hypothesis that the HERG K897T polymorphism selectively promotes arrhythmias in the setting of ischemia via alterations in channel turnover and or phosphorylation. Biochemical and electrophysiological studies will be performed in-vitro using cell lines and in-vivo using a rabbit MI model. 3) Test whether functional polymorphisms in the coding sequences and promoter regions of other cardiac genes (e.g. ion channels, beta-adrenergic receptors, connexins) predispose individuals to arrhythmias and/or heart failure progression. We hope to identify genetic predictors for the common forms of sudden cardiac death. This would allow the identification of a subpopulation of heart failure patients that would benefit most from ICD placement.